We report on the temperature dependence of electrical resistivity, Seebeck coefficient and Hall coefficient of the Fe₂VAl_1−ySi_y alloys with Si compositions y=0−0.20. While the Heusler-type Fe₂VAl (y=0) exhibits a semiconductor-like resistivity behavior, a slight substitution of Si for Al causes a sharp decrease in the low temperature resistivity and a large enhancement in the Seebeck coefficient. Substantial enhancements for the Seebeck coefficient are in reasonable accord with changes in the Hall coefficient and can be explained on the basis of the electronic structure, where the Fermi level shifts slightly from the center of the pseudogap due to the substitution of Si. In particular, the Si substitution of y=0.10 leads to a large power factor of 5.4×10⁻³ W/m K² at room temperature, which is comparable to that of conventional thermoelectric materials.